This invention relates to a method for characterizing the low end of the strength distribution for many kilometers of optical fiber in a relatively short period of time.
Measurement capabilities for optical fiber are well know. Testing of optical fiber falls into three categories: 1) optical properties, including total attenuation and loss; 2) mechanical parameters, including optical fiber size and core/cladding dimensions; and 3) optical fiber strength related properties.
Typically, the long term reliability of optical fiber depends on the strength of the glass. The strength of glass is controlled by the presence of cracks and flaws. Strong glass optical fibers may be easily damaged and their strength greatly reduced by the introduction of cracks. Cracks can be produced by chemical or mechanical means. The presence of cracks cause breakage of the glass optical fiber at a given stress. Therefore, to assess their reliability, glass optical fibers must be strength tested by applying tension to detect cracks and flaws.
Manufacturers of optical fiber usually proof test optical fibers in the plant. Proof testing optical fiber yields data to monitor the production process and gauge product improvement. However, customers may require proof testing of optical fiber to ensure that all spooled optical fibers have a strength above a minimum level necessary to handle safely without breaks. This level is set at a value, such as 50 or 100 kpsi, at which there is a reasonable probability of testing complete multi-kilometer lengths.
U.S. Pat. No. 4,148,218, issued on Apr. 10, 1979 to Knowles et al. discloses an apparatus for strength testing optical fiber. This apparatus applies a preset tension to a continuous length of optical fiber so that any flaws which result in a strength less than the preset tension will be detected by optical fiber breakage. First and second tractor assemblies are used to apply the preset tension to the optical fiber. The optical fiber is threaded into the first tractor assembly as it exits the furnace or alternatively from reels of previously drawn optical fiber. The Knowles et al. apparatus is capable of testing relatively long lengths of optical fiber, but is seriously deficient in that it is not practical for determining the actual load at failure for a range of strengths.
Static fatigue test equipment exists in the prior art. Static fatigue is time dependent, in the sense that failure will eventually occur, provided the stress exceeds some minimal value. One example of a prior art static fatigue tester requires loading a constant weight to a length of optical fiber. The weight strains the optical fiber until it breaks after a period of time. This type of device is limited in the number of optical fiber lengths that can be tested at a given time. A significant problem with this type of testing is that every length of optical fiber must be tested to destruction. Another drawback is the difficulty of testing multi-kilometer lengths in a short period of time. Testing of kilometer lengths of optical fiber over periods of years is impractical.
An example of a prior art dynamic fatigue tester involves applying an increasing tension to an individual length of optical fiber. The increasing tension is applied until the optical fiber breaks, recording an actual load at failure. Some of these testers are environmentally controlled (temperature and/or humidity). Both of the previously disclosed testers require time consuming set up and loading. For a general discussion of static and dynamic fatigue testers, see Kalish and Tariyal, "Static and Dynamic Fatigue of A Polymer-Coated Fused Silica Optical Fiber," Journal of American Ceramic Society, Vol. 61. No. 11-12, Nov. -Dec. 1978, pp 518-523.
The prior art testers identified above are particularly instructive to the extent that they appear to reflect a failure by the prior art to conceive of an apparatus to continuously strength test optical fiber. Prior art testers are unable to measure the distribution of strength of only the low strength breaks. Thus, one attribute of the present invention is that, in the preferred embodiment thereof, a method and an apparatus is provided which is particularly adapted to the task of automatically testing reels of optical fiber and detecting the actual load at failure of lower strength breaks.
It is therefore an object of the present invention to provide an apparatus for applying increasing tension to incremental lengths of an optical fiber.
Another object of the present invention to provide an apparatus for automatically indexing lengths of optical fiber.
It is another object of the present invention to apply increasing tension to an optical fiber to establish the complete range of actual strength values for a length of optical fiber.
It is another object of the present invention to provide an apparatus for measuring load at failure without destroying all of the optical fiber.